Although it is generally accepted that the arterial tree stiffens after prolonged exposure to elevated pressure, the sequence of the changes in the mechanical properties of the vessel wall which occur during the early phase of hypertension has not been fully elucidated. This study will identify these changes, document when they occur, and expose the underlying structural alterations which cause the modification of the elastic properties of the wall. Renovascular hypertension will be produced in a group of mature, miniature swine by partial constriction of one renal artery and contralateral nephrectomy (1-kidney Goldblatt model). The in vivo relationship between circumferential stress and strain will be determined for the abdominal aorta of these animals, as well as for the same vessel from a group of normotensive swine at various times ranging up to six weeks after surgery from simultaneous measurements of the intravascular pressure, and diameter and length of the vessel. The external diameter and length of the abdominal aorta of the conscious swine will be measured using an ultrasonic strain gauge. The results of these studies will be correlated with quantitative light and electron microscopic studies of aortic medial structure. Changes in the size and number of medial smooth muscle cells and in the density and orientation of collagen and elastin fibers which occur in the early phase of hypertension will be established by morphometry. This study will enable us to assess degenerative changes which occur in the arterial tissue in hypertension before these changes become irreversible by measuring the elastic properties of the vessel wall.